Mixtures of methyl acetate and methanol are generated when producing polyvinyl alcohol by way of saponification of polyvinyl acetate. U.S. Pat. No. 7,115,772 to Picard et al. discloses a process wherein a methyl acetate/methanol mixture is recovered from a mother liquor of a saponification process. The '772 patent further suggests that the mixture can be used as a feedstock in a carbonylation process to make acetic acid. It was determined, however, that methyl acetate/methanol mixtures so produced contain significant levels of acetaldehyde which are believed to have a deleterious effect on impurities generated during carbonylation. In particular, it is believed that acetaldehyde present in a carbonylation reaction mixture produces propionic acid which is difficult and expensive to remove from acetic acid in order to meet product specifications.
Acetaldehyde is particularly difficult to remove from methyl acetate/methanol mixtures in spite of its relatively low boiling point (20.9° C.). So also, methanol is difficult to separate from methyl acetate because methanol and methyl acetate form a low boiling azeotrope. Thus, in conventional operations, methyl acetate is typically hydrolyzed to acetic acid and methanol prior to further purification and/or re-use as a feedstock.
It has been unexpectedly found in accordance with the present invention that acetaldehyde removal from a methyl acetate/methanol/acetaldehyde mixture by distillation is remarkably enhanced by conducting the distillation at elevated pressure and temperature. Thus purified, the methyl acetate/methanol mixture can be directly fed to a carbonylation unit to make acetic acid without the need for more elaborate treatment.